1. Field of the Invention
The present invention relates to electrical heating of subsea pipelines. More particularly the invention relates to electrical heating with a pipe-in-pipe configuration.
2. Description of Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep, where temperatures of the water near the sea floor are in the range of 40xc2x0 F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a xe2x80x9chydrate.xe2x80x9d Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressurizing the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create operational problems and be costly in terms of lost production.
The problem of lower temperatures in pipelines has been addressed by a variety of heating methods, including electrical heating. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (xe2x80x9cDirect Impedance Heating of Deepwater Flowlines,xe2x80x9d OTC 11037, May, 1999). One electrical heating method is the pipe-in-pipe method. In one configuration of a pipeline using this method, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at the distal or remote end of a heated segment by a bulkhead. Voltage is applied between the inner and outer pipes at the proximate or electrical input end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707. Other variations of the general pipe-in-pipe method exist. The electrical power is supplied through an electrical isolating joint at the power input end of a segment of line to be heated. Alternating current, normally at about 60 Hz, is used. The voltage across the annulus is highest at the isolating joint and falls linearly to zero at the bulkhead. The current is essentially constant along the entire length of the pipe segment that is heated. Two key electrical effects, the skin effect and the proximity effect, confine the current flow largely to the annulus surfaces. Consequently, most of the current is effectively isolated from the produced fluids and the seawater around the pipeline.
In pipe-in-pipe electric heating configurations, an annulus design that electrically isolates the inner and outer pipe and provides thermal insulation and load sharing is desirable. Electrical isolation between the inner and outer pipe is needed so that the pipes will not shortout, or develop an electrical fault. Thermal insulation is advantageous because it minimizes heat loss from the inner pipe and reduces the amount of electrical current necessary to achieve the desired temperature in the inner pipe. Load sharing between the pipes helps limit the stress on the outer pipe during laying.
An annulus design that prevents complete flooding of the annulus and confines flooding caused by a breach of the outer pipe to a subsection of the annulus is needed. There is also a need for an annulus design that allows for the removal of small amounts of water that may be present in the annulus.
Towards providing these and other advantages, the present invention provides an annulus for a pipe-in-pipe electrically heated pipeline having an electrically and thermally insulating sleeve over the inner pipe in selected segments of the annulus, with a gap between the sleeve and the outer pipe. The sleeve is preferably made of polyurethane foam with an impermeable plastic skin on the outside surface. The inside pipe may be coated with fusion bonded epoxy. The weld joints in the inside pipe may be covered with half-shells, also made of foam. In parts of the annulus where greater thermal insulation is needed, more of the annulus contains the insulating sleeve. If no thermal insulation is needed, the insulating sleeve may be omitted and insulating pipe centralizers be used.
The annulus may also include water stops. A water stop includes a solid polyurethane plug that is formed from liquid polymer placed in the annulus over a seal. A second seal is present on top of the plug. The seals are preferably made of rubber having the proper hardness. Both seals may have one or more collars on top of the seal, so that water can collect around or between the collars without causing an electrical short in the annulus. A highly water-absorbent material may also be placed on top of the seal. When the plug is formed with the annulus not in an upright position, the top surface of the plug is not perpendicular to the pipe axis and an angle-correcting piece is provided between the plug and the top seal. Seals or water stops may be used in segments of the pipeline regardless of whether thermal insulation is needed. Seals may be used regardless of whether a plug is used, as the seals alone can confine movement of water in the annulus if there is not a breach of the line. Less thermal insulation may be used in the riser, particular in the upper or shallower water parts of the riser.